An optical pickup is a device for converging a light beam onto a signal recording surface of an optical recording medium such as an optical disc and detecting the return light beam reflected by the signal recording surface. Examples of wavefront aberration that occurs in the optical pickup are, for example, an astigmatism that is caused by a shape of an optical part for guiding the light beam to the optical recording medium or a deviation from a design position of the optical part; a coma-aberration that is caused by an inclination of a normal of the signal recording surface from an optical path; a spherical aberration that is caused by a variation in thickness of cover layer with which the signal recording surface is covered; or the like. Those wavefront aberrations become a cause for, for example, distorting a spot shape of the light beam which is irradiated onto the signal recording surface and deteriorating reproducing characteristics and recording characteristics. In particular, although a recording density of the optical recording medium can be improved owing to the recent realization of a short wavelength of the light beam and high resolution of an objective lens, an aberration generation amount increases. It is, therefore, demanded to correct the wavefront aberration at a high level. As an element adapted to correct the wavefront aberration of the above kind, a liquid crystal optical element (hereinbelow, called a “liquid crystal correcting element”) having a birefringent liquid crystal layer is used. Related arts regarding the liquid crystal correcting element have been disclosed in, for example, Patent Document 1 (Japanese Patent Kokai No. 2005-122828) and Patent Document 2 (U.S. Patent Application Publication No. 2005/083824).
The liquid crystal correcting element generally used has: two electrode layers which face each other; and a liquid crystal layer sealed between the electrode layers. At least one of the two electrode layers has an electrode pattern constituted by a plurality of electrode segments in order to correct a wavefront aberration component. A driving circuit can apply desired electric field distribution to the liquid crystal layer between the electrode layers by individually applying a driving voltage to the electrode segments. Since a liquid crystal molecule in the liquid crystal layer is oriented in accordance with the electric field distribution, a light transmitting medium having refractive index distribution which differs locally in accordance with an orientation state of the liquid crystal molecule is obtained. Since an optical path length of the light beam is proportional to a product of a refractive index of the light transmitting medium and a geometrical distance, the refractive index distribution which can set off the wavefront aberration can be obtained.
When driving the liquid crystal correcting element, the driving circuit calls a correction data set which has previously been stored in a nonvolatile memory and generates a driving voltage to be applied to each electrode segment in accordance with a value of the correction data set. Since the generation amount of the wavefront aberration can be predicted to a certain extent at a design stage of the optical pickup, the value of the correction data set is set, for example, in such a manner that the refractive index distribution adapted to properly correct the astigmatism component or spherical aberration component occurs in the liquid crystal layer of the liquid crystal correcting element. If the wavefront aberration is properly corrected, the distortion of the spot shape of the light beam which is irradiated onto the optical recording medium is corrected and a level of an RF signal rises, so that a using efficiency of the light beam is improved.
When the correction of the wavefront aberration is properly effected, however, there is a case where a servo is contrarily made unstable. For example, when the wavefront aberration is properly corrected, there is a case where a noise component included in a focusing error signal becomes recognizable. The noises of a radio frequency component causes an actuator for driving the objective lens to generate a heat and there is a case where the heat causes a peel-off of a reflection preventing film of the objective lens or a dissolution of the objective lens. If the wavefront aberration is corrected to such an extent that the noise component included in a focusing error signal is not recognizable, the efficiency of utilization of the light beam decreases.    Patent Document 1: Japanese patent Kokai No. 2005-122828    Patent Document 2: U.S. Patent Application Publication No. 2005/083824 (the laid-open publication of U.S. Patent Application corresponding to Patent Document 1)